Digital readout instrument employing transducer and double d.c. power supply

ABSTRACT

A digital readout thermometer employing a resistance bridge with a temperature sensitive resistance such as a thermistor in one leg of the bridge and including a motor driven adjustable resistance or potentiometer for rebalancing the bridge. A servo amplifier and d.c. servo motor servo drive a digital counter to provide a reading indicative of the temperature. The circuit employs a d.c. power supply for the bridge circuit and a second d.c. power supply cooperating with the first power supply for furnishing the servo circuit with opposite polarity voltages whereby the digital counter will operate in both directions.

United States Patent I 11 91 [11] 3,765,244 Brzezinski Oct. 16, 1973 [5DIGITAL READOUT INSTRUMENT 3,441,846 4/1969 PCII'Ol'lllOS 73/362 AREMPLOYING TRANSDUCER AND DOUBLE g Z;

, ees on POWER SUPPLY 3,254,533 6/1966 Tongret 1 73/362 AR [76]Inventor: Janusz Brzezinski, c/o John E. 3,469,449 9/1969 Keller 73/362AR Wagner, Patent Atty., 1041 East 7 Green St., Pasadena, Calif. 91106Primary Examiner-Richard C. Queisser I Assistant Examiner-FrederickShoon [22] Filed 1971 Attorney-John E. Wagner [21] Appl. No.: 135,338

Related US. Application Data ABSTRACT l [63] continuationdmpart of SenNo. 32,142, April 27, A d1g1tal readout thermometer employing aresistance 1970 abandone bridge with a temperature sensitive resistancesuch as a thermistor in one leg of the bridge and including a [52] [1,3,(1|, 673/362 R, 323/75 H 323/75 R motor driven adjustable resistance orpotentiometer [51] Int. Cl. 001k 7/24 for febalancing the bridge- AServo amplifier and [58] Field 6! Search 73/362 AR; 323/75 11,- t Servomotor Servo drive a digital counter to provide a v 323/75 N readingindicative of the temperature. The circuit employs a d.c. power supplyfor the bridge circuit and a [56], References Ci d second d.c. powersupply cooperating with the first UNXTED STATES PATENTS power supply forfurnishing the servo circuit with op- 3 545 273 [2/1970 F1) t 73,362 ARposite polarity voltages whereby the digital counter r1 erge a 3,287,97811 1966 Knudsen 73 362 AR will operate m both drecnons' 4 Claims, 2Drawing Figures (I POSITIVE POWER SUPPLY M REGULATOR :16 5 ihH/iw u-BRIDGE AMPLIF- TRANSDUCER IER l 1 [G i 45 L iii/iii bggGATVE SUI U?REGULATOR PATENTEUum 16 m5 SHEEI 1 [1F 2 MNJ PATENTEUIJBI 16 ms SHEET, 2BF 2 DIGITAL READOUT INSTRUMENT EMPLOYING TRANSDUCER AND DOUBLE D.C.POWER SUPPLY RELATED INVENTIONS This is a continuation in part of myapplication, Ser. No. 32,142 filed Apr. 27, I970, and now abandoned.

BACKGROUND OF THE INVENTION With the advent of miniaturized electroniccircuits and solid state temperature sensing devices the development ofan all electronic thermometer for human use is possible. As a result, anumber of such devices have been produced. Generally they employthermistors as the temperature sensing element, a Wheatstone bridge, apower supply and a meter for reading bridge unbalance as an indicator oftemperature. Such thermometers using a meter give only a transitoryreading and incur the problems associated with accurately reading ameter. Therefore the need for a digital readout thermometer has beenrecognized.

A number of digital readout electronic thermometers have been developedas typified by US. Pat. Nos. 3,469,449; 3,408,568; 3,469,449; and3,441,846. Characteristically the di'gital readout thermometersdeveloped to date' require complex circuitry such as synchronousdetectors, phase frequency or voltage references, chopper circuits,clocks or other circuits which are subject to drift and requirecompensation of various types.

BRIEF STATEMENT OF THE INVENTION In fulfilling the need for an effectivedigital readout thermometer, I have invented an electronic thermometeremploying a resistance bridge with a temperature sensor in one leg ofthe bridge, an adjustable potentiometer for balancing the bridge, aservo amplifier and motor and digital counter.

The device employs only direct current sources and simple substantiallylinear circuits throughout.

Likewise I have utilized a pair of simple easily replaceable batteriesas the power supplies for the thermometer. The power supplies are sodesigned and arranged that the bridge circuit supply voltage isregulated while the servo circuit is unregulated but bipolar to providebidirectional movement of the digital readout device. B

One feature of the invention resides in the power supply circuit wherebya low battery or dead battery condition is indicated and incorrectreading will not occur.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a block diagram of thedigital meter of the invention;

FIG. 2 is a schematic diagram of the electric circuit of the meter ofinvention.

DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawing,FIG. 1. An electronic thermometer or pressure sensor in accordance withthis invention is shown as including a pair of power supplies 1 and 2,one a positive potential supply associated with a battery 46 and anegative potential supply associated with battery 45. The two powersupplies 1 and 2 are connected to a common terminal or system groundlead G.

The power supply 1 is connected across bridge circuit 4 which has as oneinput a pressure or temperature transducer 3. The bridge circuit 4includes an adjustable element 14 as a variable arm. The adjustableslider of the potentiometer or adjustable resistance element 13 isconnected via resistance 11 to an amplifier 5 which in turn is connectedto a servo power amplifier 6 designed to provide sufficient current todrive the mechanical components of the system, including a dc. servomotor 7, gear trains 8 and 9, and a digital counter 10. System groundfor the servo motor 7 is provided through resistance 22 to ground leadG. The servo motor is coupled mechanically through gear train 9 to drivethe counter 10 and through gear train 8 to drive the wiper ofpotentiometer 13. A feedback loop from the servo motor 7 is providedthrough resistance 12 to the input of the amplifier 5. Thus, a completeposition servo loop is provided by the following components:potentiometer l3, resistance 11, amplifiers 5 and 6, servo motor 7, andresistance 12 and 22. Note particularly in FIG. 1 that two batteries 45and 46 are connected in series with the ground lead G at the center tapbetween the negative terminal of battery 46 and the positive terminal ofbattery 45. The battery 46 supplies the potential across the bridge.This potential is regulated by supply 1. The batteries 45 and 46,together with their respective negative and positive power supplyregulators 2 and 1, provide regulated potentials to the amplifier 5 andthe same batteries provide unregulated opposite potentials via leads 23and 24 to the servo power amplifier 6. Inasmuch as the servo poweramplifier is in the null seeking servo loop, regulated power suppliesare not required. Likewise, the independence of the bridge circuitsupply from the drive motor supply as afforded by the circuits 1 and 2minimizes any node voltage changes at the bridge due to motor currentrequirements.

The details of the circuit of this invention are more clearly seen inFIG. 2 showing the electrical schematic of the device. It may be seenthat in addition to the basic elements of the invention shown in FIG. 1,the circuit employs a pair of switches 25 and 26 which serve to connectthe batteries 45 and 46 to the remainder of the circuit. These twoswitches constitute the main power switch for the circuit. Also, thepositive power supply 1 of FIG. 1 in actuality constitutes a voltageregulator employing a series resistor 27, a shunt to ground capacitor 29and similarly shunted to ground Zener diode 31. The negative powersupply 2 of FIG. 1 likewise is made up of a series resistance element28, a shunt capacitor 30 and a shunt Zener diode 32. This configurationis well known in the art as an effective voltage regulator. The supplyregulator is made up of the components identified above supplying aregulator voltage of six volts to the bridge circuit 4 made up ofresistance elements 15, 16, 17 and 18 connected in the conventionalWheatstone bridge configuration. The transducer 3 of FIG. 1 constitutesa temperature sensitive resistance for thermistor 14 while thepotentiometer 13 of FIG. 1 includes resistance element 21 and wiper 50.A capacitor 33 connected across the resistance 16 and the thermistor 14serves to improve the signal to noise ratio. They act as an RF filter.

The amplifier 5 is designed to amplify the differential voltage, if any,appearing at the terminals 20 and 51 in the same manner as aconventional meter in a Wheatstone bridge. The output voltage, whetherof positive or negative polarity with respect to ground, is passedthrough diode 34 or 35, respectively, at the input to the two-stageservo amplifier including transistors 38, 41, 43 and 44 with theirassociated resistance elements 37, 39, 40 and 42. All elements oftransistors 28, 41, 43 and 44 and diodes 34, 35 and resistors 36, 37,39, 40 and 42 constitute the servo amplifier of FIG. 1.

Viewing now FIGS. 1 and 2, it is apparent that l have an exceedinglysimple circuit for providing digital readout of a sensed condition, suchas, human body temperature, employing basically linear circuitry. Thecircuit employs two batteries of similar nature, such as a conventional9 volt battery designed primarily for transistor radio use. Thecircuitry is arranged such that one such battery provides the potentialfor the Wheatstone bridge after being regulated by the supply regulatorl. The first and second batteries cooperate to provide unregulatedbi-directional voltage for operating the servo circuitry wherebi-directionality is required but regulation is of less significance.

Employing the power supply circuit as described above, certain uniquefeatures are possible in this device. First, the measurement circuitryis supplied by one battery only so that variations in terminal voltageof that battery will not affect the bridge balance. The amplifiercharacteristically has a minimum operating voltage and, if the battery46 supplying potential to the bridge and additionally to the amplifier 5has insufficient residual power, no error signal from amplifier 5 willbe applied to the servo system and the digital readout will not move. Ifthe battery 45 has insufficient power for operation while battery 46 isadequate, the bridge circuit and amplifier 5 will be operative and anerror correction signal driving the servo motor full to one end will beintroduced into the servo circuit. Depending upon the connection of theservo motor, the digital counter will run either to 90 or 110, both ofwhich are outside of the range of correct values and obvious to the userthat the device is out of service.

Given the above power supply circuit, the instantaneous testing of theoperativeness of the system can easily be accomplished merely by closingthe switches 25 and 26 when the sensor is under ambient temperature(90-110). If the digital counter runs to a minimum value such as 90, itclearly indicates that the circuit is operative since there was clearlysufficient power capability in the battery to operate the bridge circuitin the battery 46 and sufficient power in the circuit in the battery 45to drive the servo motor. The system is therefore fail-safe and any lackof battery power produces a clearly impossible reading.

The circuit is designed to use such components which are almostinsensitive to ambient temperature. This means approximately no errordue to ambient temperature.

The preferred embodiment of this invention is illustrated in the drawingand described above as employing regulated power supplies and with thebridge circuit connected across battery 46. It must be recognized thatthe bridge circuit may instead be connected across battery 45 or acrossboth batteries 45 and 46 with similar performance. If unregulated powersupplies are used, the thermometer will of course work but its accuracyis decreased.

The above described embodiments of this invention are merely descriptiveof its principles and are not to be considered limiting. The scope ofthis invention instead shall be determined from the scope of thefollowing claims including their equivalents.

I claim:

1. A battery powered portable system for digitally displaying a measuredparameter comprising:

A transducer having electrical characteristics varying with theparameter to be measured;

A rebalancing bridge including said transducer as the leg of saidbridge;

An adjustable leg of said bridge capable of rebalancing the bridge tocompensate for variation in the bridge balance because of saidtransducer;

A first battery;

Means for applying the potential from said first battery across saidbridge to provide operating potential therefore;

An amplifier connected to said bridge to amplify the potential from saidbridge related to the position of said adjustable leg of said bridge;

A second battery;

Means for applying power from said first and second batteries to saidamplifier whereby said amplifier provides an output signal related inmagnitude and polarity to the degree and direction of adjustment of saidadjustable leg;

Servo means driven by said amplifier including a digital display devicefor displaying a visual indication of the parameter measured by saidtransducer;

Said servo means including a feedback connection between said servomeans and said bridge to reduce the signal produced by said amplifier tosubstantially zero while said indicator matches in reading the parametermeasured by said transducer;

and I Said first and second batteries connected to said servo means todrive said servo means;

Including voltage regulator means for applying regulated potential fromsaid first and second batteries comprising a pair of Zener diodenetworks furnishing regulated opposite potentials to said amplifierwhereby the output of said amplifier is a function of the differentialpotential at its signal input and insensitive to supply potentialvariations.

2. The combination in accordance with claim 1 including means forapplying unregulated opposed polarity power from said first and secondbatteries to said servo means.

3. A portable battery powered electronic thermometer comprising:

a temperature sensitive resistance;

a resistance bridge including said temperature sensitive resistance inone leg thereof;

said bridge also including a potentiometer for balancing said bridge;

first battery power supply means for applying a potential across saidbridge;

amplifier means connected to said balancing potentiometer for amplifyingany bridge unbalance potential;

a digital readout device connected to said amplifier means for recordinga temperature as sensed by said temperature sensitive resistance as ameasure of the balancing potentiometer position;

motor means for driving said digital readout device;

a feedback connection means between said motor means and said amplifiermeans for completing a servo loop between said balancing potentiometerand said digital readout device;

means connecting said first power supply means to provide power to drivesaid motor means in one direction; and,

second battery power supply means connected to said motor means fordriving the readout device in the opposite direction;

said first battery power supply means includes voltage regulator meansfor maintaining a substantially constant voltage across said bridge; andincluding amplifier means for amplifying a difference voltage related tothe unbalance of the bridge wherein said first and second battery powersupply means provide regulated voltage to said amplifier means.

4. A portable battery powered electronic thermometer comprising:

a temperature sensitive resistance;

a resistance bridge including said temperature sensitive in one legthereof;

said bridge also includinga potentiometer for balancing said bridge;

first battery power supply means for applying a potential across saidbridge;

amplifier means connected to said balancing potenti ometer foramplifying any bridge unbalance poten- 6 tial;

a digital readout device connected to said amplifier means for recordinga temperature as sensed by said temperature sensitive resistance as ameasure of the balancing potentiometer position;

motor means for driving said digital readout device;

a feedback connection means between said motor means and said amplifiermeans for completing a servo loop between said balancing potentiometerand said digital readout device;

means connecting said first power supply means to provide power to drivesaid motor means in one direction; and,

second battery power supply means connected to said motor means fordriving the readout device in the opposite direction; wherein said firstbattery power supply means includes voltage regulator means formaintaining a substantially constant voltage across said bridge; whereinsaid second battery power supply means is connected to drive said servomotor fully in one direction in the absence of power from said firstbattery power supply means to indicate the inoperativeness of said firstbattery power supply means.

* III =l=

1. A battery powered portable system for digitally displaying a measuredparameter comprising: A transducer having electrical characteristicsvarying with the parameter to be measured; A rebalancing bridgeincluding said transducer as the leg of said bridge; An adjustable legof said bridge capable of rebalancing the bridge to compensate forvariation in the bridge balance because of said transducer; A firstbattery; Means for applying the potential from said firsT battery acrosssaid bridge to provide operating potential therefore; An amplifierconnected to said bridge to amplify the potential from said bridgerelated to the position of said adjustable leg of said bridge; A secondbattery; Means for applying power from said first and second batteriesto said amplifier whereby said amplifier provides an output signalrelated in magnitude and polarity to the degree and direction ofadjustment of said adjustable leg; Servo means driven by said amplifierincluding a digital display device for displaying a visual indication ofthe parameter measured by said transducer; Said servo means including afeedback connection between said servo means and said bridge to reducethe signal produced by said amplifier to substantially zero while saidindicator matches in reading the parameter measured by said transducer;and Said first and second batteries connected to said servo means todrive said servo means; Including voltage regulator means for applyingregulated potential from said first and second batteries comprising apair of Zener diode networks furnishing regulated opposite potentials tosaid amplifier whereby the output of said amplifier is a function of thedifferential potential at its signal input and insensitive to supplypotential variations.
 2. The combination in accordance with claim 1including means for applying unregulated opposed polarity power fromsaid first and second batteries to said servo means.
 3. A portablebattery powered electronic thermometer comprising: a temperaturesensitive resistance; a resistance bridge including said temperaturesensitive resistance in one leg thereof; said bridge also including apotentiometer for balancing said bridge; first battery power supplymeans for applying a potential across said bridge; amplifier meansconnected to said balancing potentiometer for amplifying any bridgeunbalance potential; a digital readout device connected to saidamplifier means for recording a temperature as sensed by saidtemperature sensitive resistance as a measure of the balancingpotentiometer position; motor means for driving said digital readoutdevice; a feedback connection means between said motor means and saidamplifier means for completing a servo loop between said balancingpotentiometer and said digital readout device; means connecting saidfirst power supply means to provide power to drive said motor means inone direction; and, second battery power supply means connected to saidmotor means for driving the readout device in the opposite direction;said first battery power supply means includes voltage regulator meansfor maintaining a substantially constant voltage across said bridge; andincluding amplifier means for amplifying a difference voltage related tothe unbalance of the bridge wherein said first and second battery powersupply means provide regulated voltage to said amplifier means.
 4. Aportable battery powered electronic thermometer comprising: atemperature sensitive resistance; a resistance bridge including saidtemperature sensitive in one leg thereof; said bridge also including apotentiometer for balancing said bridge; first battery power supplymeans for applying a potential across said bridge; amplifier meansconnected to said balancing potentiometer for amplifying any bridgeunbalance potential; a digital readout device connected to saidamplifier means for recording a temperature as sensed by saidtemperature sensitive resistance as a measure of the balancingpotentiometer position; motor means for driving said digital readoutdevice; a feedback connection means between said motor means and saidamplifier means for completing a servo loop between said balancingpotentiometer and said digital readout device; means connecting saidfirst power supply means to provide power to drive said motor means inone direction; aNd, second battery power supply means connected to saidmotor means for driving the readout device in the opposite direction;wherein said first battery power supply means includes voltage regulatormeans for maintaining a substantially constant voltage across saidbridge; wherein said second battery power supply means is connected todrive said servo motor fully in one direction in the absence of powerfrom said first battery power supply means to indicate theinoperativeness of said first battery power supply means.